Improved juju get help
Merge pull request #1167 from dimitern/lp-1392544-better-docs-for-juju-get-in-master

Improved juju get help

According to this bug http://pad.lv/1392544 the output
of "juju get" was somewhat confusing because the "default"
field in the output looks the same as "default" field in
the charm config definition, but they mean different things.

This PR ports the approved #1166 into master.

(Review request: https://reviews.vapour.ws/r/478/)